RE: sail size for Go 139
It could work, but there are alot of other things that would need to be
figured out (sailor weight/windspeeds/water surface conditions/sailor skills) before making a positive recommendation.
Basically it sounds like you want super early planing on a smaller narrower board, and that doesn't always work unless you are very light in weight or have really good pumping skills.
Width of the board seems to have more positive effect on marginal planing than sail size.
Combine a good wide (85 cm +) board and a 10.0 m2 rig and you are going to get good marginal planing.
The GO 139 (@ only 80 cm wide) isn't going to be very easy to pump onto a plane with a 10.0 m2 rig.
Also, most 10.0 m2 rigs are race sails with several cams and designed for formula boards and formula racing which entails sailing about 1.5-2.0 m2 overpowered to get the good upwind and downwind speeds and angles formula boards are known for.
If you get a smaller "free race" type sail, with more static draft and low end "grunt" you would save quite a bit of weight (shorter mast, shorter boom, fewr cams or "no cams", etc. and this could mean that a 9.0 m2
would actually be alot less expenisve, and get you going just as early.
So, my "best guess" here is that the initial answer would be "NO" in my opinion!
Hope this helps,